1. Field of the Invention
This invention relates to a method of growing a .beta.-SiC layer on a silicon substrate. More specifically, this invention concerns a method of growing a single crystalline .beta.-SiC layer on a silicon substrate, in which all processes are performed heating the substrate at temperatures below 1000.degree. C. A heterojunction formed between a .beta.-SiC layer and a silicon substrate has a promising application for high-speed and high-power transistors working at a high temperature.
2. Description of the Related Art
Compared with silicon, silicon carbide SiC has a wide energy gap of 2.2 eV and has other distinctive features in heat resisting property, thermal conductivity, break down voltage, saturated electron drift velocity, etc. Silicon carbide has several crystalline structures such as 3C-SiC (cubic system, called .beta.-SiC) and 6H-SiC (hexagonal system, called .alpha.-SiC). Especially, .beta.-SiC having an energy gap of 2.2 eV is popular and has been the subject of investigation for developing a high temperature FET (Field Effect Transistor), a light emitting diode of green rays, and a heterojunction bipolar transistor using the heterojunction between a silicon substrate and a single crystalline .beta.-SiC layer grown thereon.
In order to grow a single crystalline .beta.-SiC layer on a silicon substrate, several methods have been investigated. Among them, a CVD method is most popular, in which the substrate is heated up at a temperature around 1300.degree. C. using, for example, propane C.sub.3 H.sub.8 and silane SiH.sub.4 (or trichlorosilane SiHCl.sub.3) gases as source material gases and a hydrogen gas as a carrier gas.
The problem encountered in the above method is that the grown .beta.-SiC layer shows deterioration in a crystalline structure when a thickness of the grown .beta.-SiC layer becomes greater than 1 .mu.m. The grown layer does not show a .beta.-SiC epitaxial layer or a single crystalline structure of .beta.-SiC but rather a poly-crystalline structure. Therefore, it can not be used for fabricating a semiconductor device using a thick .beta.-SiC layer on a silicon substrate. The main reason for the deterioration of crystalline structure is due to a lattice mismatch between the silicon and .beta.-SiC crystals. Because the lattice constant of silicon is 5.430 .ANG. and that of .beta.-SiC, 4.358 .ANG., the difference being about 20% therebetween.
In order to improve the crystalline quality of the grown .beta.-SiC layer, a method of growing an intermediate layer by carbonizing silicon substrate between the silicon substrate and the .beta.-SiC layer has been proposed. The intermediate layer is comparatively thin and relieves the stress between the silicon substrate and the grown single crystalline .beta.-SiC layer. The intermediate layer having thickness of several tens .ANG. to a few hundred .ANG. acts as a buffer layer for the epitaxial .beta.-SiC layer.
The method of growing the buffer layer of the prior art includes the steps of heating up the silicon substrate in a temperature range between 1300.degree. C. to 1350.degree. C. and introducing a propane C.sub.3 H.sub.8 gas as a source material gas. When a thickness of the buffer layer is sufficiently thin, the grown .beta.-SiC layer on the buffer layer shows a single crystalline .beta.-SiC structure and it does not become a serious problem in forming a heterojunction between the silicon substrate and the .beta.-SiC epitaxial layer grown on the buffer layer. Though it has become possible to decrease the growth temperature of the single crystalline .beta.-SiC on the buffer layer to a temperature less than 1000.degree. C., the temperature 1300.degree. to 1350.degree. C. for growing the buffer layer is extremely high when the fact is taken into consideration that the melting point of silicon is about 1400.degree. C.
The high temperature of growing the buffer layer beneath the single crystalline .beta.-SiC layer in the prior art has bad influences on the silicon substrate such as extension of an impurity diffusion region, mechanical deformation of the substrate, etc. Many efforts have been directed to decrease the growth temperature of the buffer layer or to eliminate the process of growing the buffer layer by growing the .beta.-SiC epitaxial layer directly on the silicon substrate.